1. Field of the Invention
The present invention relates generally to photonic band gap fibers, and particularly to a method of making photonic band gap fibers.
2. Technical Background
Traditionally optical waveguide fibers have used total internal reflection to guide the propagation of an optical signal. Optical waveguide fibers that rely upon total internal reflection for the transmission of optical signals typically have a core region and a cladding region. The core region is the portion of the optical waveguide fiber that the optical signal propagates within. Generally, the core region of an optical waveguide fiber relying on total internal reflection to guide the propagation of an optical signal has a higher index of refraction than surrounding cladding region.
Optical waveguide fibers that rely upon total internal reflection in order to guide the propagation of optical signals have of inherent limitations. Among these are relatively high dispersion and attenuation of the optical signal, and relatively low upper limits on the power of the optical signal.
Photonic band gap (PBG) fibers are photonic crystals that have a structure in which the refractive index varies periodically in 2 dimensions, (the x-y plane, where the z-coordinate is the longitudinal axis of the fiber), with a period of the order of an optical wavelength. Photonic band gap fibers may offer a better performance than total internal reflection optical waveguide fibers with regard to dispersion, attenuation and signal power.
A photonic crystal is a structure having a periodic variation in dielectric constant. The periodic structure of the crystal may be one, two or three-dimensional. A photonic crystal allows light of certain wavelengths to pass through it and prevents the passage of light having certain other wavelengths. Thus photonic crystals are said to have allowed light wavelength bands and band gaps that define the wavelength bands that are excluded from the crystal. A review of the structure and function of photonic crystals is found in, Joannopoulus et al., xe2x80x9cPhotonic Crystals: putting a new twist on lightxe2x80x9d, Nature vol. 386, Mar. 13, 1997, pp. 143-149.
A two-dimensional photonic crystal having certain geometries and effective indices of refraction may produce a photonic band gap fiber in which the optical signal propagates in either air or vacuum. Use of a 2 dimensional photonic crystal as an optical fiber is discussed in, Birks et al., xe2x80x9cFull 2-D photonic band gaps in silica/air structuresxe2x80x9d, Electronic Letters, Vol. 31 (22), Oct. 26, 1995, pp. 1941-1943. Through Bragg diffraction, these structures can support a series of optical resonances, band gaps and allowed states.
An optical waveguide fiber in which the optical signal propagates in air or vacuum is of great interest in the field of telecommunications. This interest arises because optical waveguide fibers in which the optical signal propagates in air or vacuum offer lower dispersion, lower attenuation of the optical signal being carried and have a near zero nonlinear refractive index. Compared to air guiding photonic band gap fibers, current total internal reflection fibers have a limited operating regime.
Recent theoretical work has indicated that large void-filling fractions are required for optical waveguide fibers to propagate light in a low index of refraction core utilizing the photonic band gap effect. The low index of refraction core typically includes an evacuated or air filled passageway in which the light is guided. The void-filling fraction is a function of the ratio of the diameter of the passageways to the center to center spacing, or pitch, of the passageways. Equation 1 is the mathematical expression for the void-filling fraction of a photonic band gap fiber, vf.                     vf        =                                            π                              2                ⁢                                  3                                                      ⁡                          [                              d                Λ                            ]                                2                                    (        1        )            
where
vf is the void-filling fraction;
d is the diameter of internal passageways; and
xcex9 is the distance between the centers of adjacent passageways or pitch.
Photonic band gap air-guiding fibers with a void-filling fraction of 0.42 have been fabricated using a stack and draw process. A detailed description of the stack and draw process may be found in R. F. Cregan, Single-Mode Photonic Band Gap Guidance of Light in Air, SCIENCE, vol. 285, pp. 1537-39 (1999).
Optical waveguide fibers having large void-filling fractions are obtained by drawing photonic crystal preforms having large void-filling fractions into optical waveguide fibers using conventional optical waveguide fiber making techniques.
Photonic crystal preforms have been made using the stack and draw method and the extrusion method. The stack and draw method involves arranging glass capillary tubes into an array having desirable macroscopic cross-sectional properties and then reducing the cross section of the preform. Typically the preform is either forced through a die or drawn to reduce the cross section. Preforms made according to the stack and draw process are categorized as either close-packed arrays or non-closepacked arrays. A close-packed array is an array of capillary tubes where the capillary tubes support one another. A non-close-packed array is an array of capillary tube wherein spacers or jigs are placed in the array thus spacing the walls of the capillary tubes apart.
Making optical waveguide fibers with a high void-filling fraction with a small pitch is difficult.
There is a need for a method of making preforms for making photonic band gap fibers that is repeatable, versatile, and adaptable to a manufacturing environment.
One aspect of the present invention is a method for making photonic band gap fibers including the step of making a photonic crystal preform having multiple longitudinal passageways. The photonic crystal preform is then etched and drawn into a photonic band gap fiber. In another aspect, the present invention includes an apparatus for etching a preform having a plurality of passageways. The apparatus includes a reservoir containing an etching agent. A heater is thermally coupled to the reservoir. A circulator having an input line and an delivery line is located so that the input line is connected to the reservoir and circulator draws etching from the reservoir and directs it to a nozzle connected to the delivery line of the circulator. The etching agent is directed by the nozzle into the passageways of the preform. The apparatus also includes a receptacle located to collect the etching agent as it exits the passageways. A return line is connected to the receptacle, and the etching agent flows through the return line and is returned to the reservoir.
In another aspect, the present invention includes a method for making photonic band gap fibers includes the steps of first assembling a number of glass tubes into a bundle. The bundle is then formed into a photonic crystal preform having a number of passageways by reducing the cross-section of the bundle. Next, one of the passageways of the photonic crystal preform is enlarged by flowing an etching agent through it. After a predetermined time has passed, the flow of the etching agent is stopped. After the etching agent is stopped flowing through the passageway, the photonic crystal preform is cleaned to remove any remaining liquid etching agent. A photonic band gap fiber is then made from the etched photonic preform. Typically, the photonic band gap fiber is made from the preform by traditional fiber drawing methods.
An advantage of the present invention is that preforms can be made which result in void-filling fractions on the order of 0.82 and greater.
Another advantage of one embodiment of the present invention is that special jigs are not required to make a preform having a large void-filling fraction.
Another advantage of the present invention is that it provides a relatively easy way to insert a large passageway in the structure of the preform and hence the resulting photonic band gap fiber. This follows from the observation that in an interior passageway or channel any surface with a positive radius of curvature, with respect to the wall of the passageway, e.g., a protrusion from the wall surface, has a greater etch rate than a flat surface. Furthermore, the etch rate of a flat surface is greater than that of a surface having negative curvature, e.g., a depression. Therefore, instead of making a preform with a large central passageway, a task that has proven difficult, a smaller passageway possessing wall of opposite curvature from the remaining passageways may be created in the preform. When the passageways are simultaneously exposed to an etching agent the desired cross-sectional shape of the passageway is realized while the void-filling fraction of the remaining passageways increases.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.